The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and to a non-aqueous electrolyte secondary battery using the same.
Recently, electronic devices have rapidly been becoming portable and cordless, and there is accordingly a demand for secondary batteries that are small-sized and light-weight but have high energy density for use as the driving power sources of such devices. From such viewpoint, non-aqueous type secondary batteries, particularly lithium secondary batteries, are expected to be suited for use as the power sources for electronic devices, since they provide high voltage and high energy density.
Among such lithium secondary batteries, lithium ion secondary batteries, which use lithium cobaltate as a positive electrode active material and use carbon material capable of intercalating and deintercalating lithium ions as a negative electrode active material, have been developed and commercialized.
The operating potential of lithium cobaltate is approximately 4 V, relative to lithium metal. Thus, the voltage of such batteries becomes higher. Also, the negative electrode includes such carbon material as the negative electrode active material, thereby making use of intercalation reaction of lithium ions into the carbon material. Therefore, lithium dendrites, which occur when lithium metal is used as the negative electrode active material, are unlikely to form. This has made it possible to significantly resolve problems with respect to degradation of charge/discharge efficiency and safety.
On the other hand, in view of the estimated amount of cobalt deposits and the cost of cobalt, and further in terms of developing lithium ion secondary batteries with higher energy density, lithium nickel composite oxides have been under development as a substitute for lithium cobaltate.
Such lithium nickel composite oxides are used in large-sized lithium ion batteries for power storage and electric vehicles, which are required to offer long-term durability and safety.
However, non-aqueous electrolyte secondary batteries using a conventional lithium nickel composite oxide as the positive electrode active material may cause deterioration of life characteristics resulting from an increase in internal impedance, for example, when they are stored in a high-temperature environment. This increase in internal impedance leads to degradation of voltage characteristics and a significant reduction in the amount of energy the battery produces. Thus, suppressing the increase in internal impedance is an important problem to be solved in batteries using a lithium nickel composite oxide as the positive electrode active material, for example, batteries for electric vehicles, which are required to provide high output.
Meanwhile, lithium cobaltate and lithium nickelate have been researched as follows. For example, regarding the use of lithium cobaltate as the positive electrode active material, there has been proposed to provide aluminum oxide (Al2O3) particles on the surface of lithium cobaltate, in order to improve thermal stability and cycle characteristics during charging without lowering the discharge capacity and charge capacity of the battery (see Japanese Laid-Open Patent Publication No. 2001-143703). There has also been proposed to coat the surface of lithium nickelate with lithium carbonate, to improve the discharge characteristics of batteries including lithium nickelate as the positive electrode active material after high-temperature-storage (see Japanese Laid-Open Patent Publication No. Hei 7-245105).
However, there is a problem with the invention of Japanese Laid-Open Patent Publication No. 2001-143703. The problem is that since the active material is coated with aluminum oxide, lithium ions move slowly in the electric double layer formed in the electrolyte near the electrode, thereby resulting in deterioration of output characteristics (high-rate discharge characteristics) among initial characteristics.
Also, with respect to the invention of Japanese Laid-Open Patent Publication No. Hei 7-245105, the present inventors have found that the capacity improves when batteries are discharged at normal discharge rates after high temperature storage, but that there is still a problem in terms of high-rate discharge characteristics. This problem is thought to be caused by an increase in impedance during storage.
It is therefore an object of the present invention to provide a positive electrode active material capable of improving discharge characteristics, particularly output characteristics even after high-temperature storage, and a manufacturing method thereof, as well as a non-aqueous electrolyte secondary battery using such a positive electrode active material.